Apple is working its way toward releasing OS X Mountain Lion 10.9.5, very likely the last non-security update for the 10-month old operating system.

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Apple is working its way toward releasing OS X Mountain Lion 10.9.5, very likely the last non-security update for the 10-month old operating system.

Apple issued a new build of the beta of 10.9.5 on Tuesday, according to 9to5Mac.com. The build, the fourth in a series for 10.9.5, was identified as "13F18."

While Apple hasn't said -- it never does -- when it will release Mavericks 10.9.5, it's almost certain to be the last update that includes non-security bug fixes, the kind of performance and reliability improvements that Mac owners hunger for, or changes to existing features in the OS.

Both Lion (OS X 10.7) and Mountain Lion (10.8) reached x.x.5 and no further, for instance.

Unless the Cupertino, Calif. company drastically revamps its practices, once Mavericks 10.9.5 appears on Apple's servers the operating system will only receive security fixes, patches for vulnerabilities that researchers report to Apple or the firm's own security team roots out.

Last year, when OS Mountain Lion -- aka OS X 10.8 -- was in its final stages, Apple shipped the last non-security update -- designated 10.8.5 -- on Sept. 12. 2013. Apple delivered the second-to-last Mountain Lion update on June 4, 2013, or more than three months before 10.8.5.

By that schedule -- and Mavericks 10.9.4's release on June 30, 2014 -- Apple won't ship 10.9.5 until Oct. 8, 2014. That seems unlikely, given that the company will release the final of Mavericks successor, Yosemite or OS X 10.10, later that month, probably on Oct. 22.

Instead, it is much more likely that Apple will deliver Mavericks 10.9.5 some time in September.

More important than the delivery date for the last non-security Mavericks refresh is the fact that the same day will also mark the end of security patches for 2011's Lion, or OS X 10.7.

Last year, Apple served the final security update for OS X 10.6, better known as Snow Leopard, on the same day it issued Mountain Lion 10.8.5. Apple also pushed out the last patches for Safari 5, the version that runs on Snow Leopard, on Sept. 12, 2013.

Because Apple now appears to be supporting -- via security updates -- only "n", "n-1" and "n-2," where "n" is the current edition, the debut of Yosemite will tip Lion (which will then be "n-3") into the retirement bin.

Under the formula, Yosemite would be "n," Mavericks "n-1" and Mountain Lion "n-2."

Lion shipped in July 2011, cost $29.99 and was the first to be distributed through the Mac App Store.

There's no guarantee that Apple will stop supporting Lion with security fixes; last year it added "n-2" to its support list, and it's possible it will expand that support even more to "n-3."

The problem, of course, is that Apple, unlike Microsoft and other major software vendors, does not spell out its support policies. Instead, it leaves users guessing about when their operating systems will fall off the support list. "Let's face it, Apple doesn't go out of their way to ensure users are aware when products are going end of life," said Andrew Storms, director of DevOps at security company CloudPassage, in a December 2013 interview about Snow Leopard support questions.

If Apple does put Lion out to the savannah, it will have dropped the OS after just three years. That's less than a third of the decade Microsoft currently promises to support a Windows edition. (However, there are signs that Microsoft may modify that policy, at least for consumers.)

Halting patches for OS X Lion customers would, of course, be painful -- and potentially risky -- for them, but Apple probably won't blink an eye. As of July, Lion accounted for just 10% of all versions of OS X in use, according to metrics company Net Applications.

By the end of September, Lion's user share will be about 8%, assuming it continues to drop at the rate it has over the last six months. That will be less than half of Snow Leopard's share when Apple pulled its plug last year.